[R-390] Field Change No. 7, R390A

[David Wise]

I also installed FC7, and like Jim found that it decreased
the sensitivity, even after adjustment of the IF gain
for maximum S/N.

I removed it.  If I acquired a radio or module
that had it, I would remove it.

We've never found the original documentation
explaining the reason for FC7.  After some thought
about the target ("radios in supplemental spaces"),
I believe it was meant for less-important R-390As
so they wouldn't interfere with more-important
ones.  It obviously does not apply :-)

73,
Dave Wise

> -----Original Message-----
> From: Jim M. [mailto:jamesmiller20@worldnet.att.net]
> Sent: Thursday, January 02, 2003 7:27 AM
> To: Tom Warren
> Cc: Morgan, Roy; r-390@mailman.qth.net
> Subject: Re: [R-390] Field Change No. 7, R390A
>=20
>=20
> I installed the mod. to my R-390a last year to see what would=20
> happen.  I
> found that it decreased the sensitivity noticeably.  By that=20
> I mean that
> a 40 dB test signal now read 30 dB on the carrier meter.  Not an exact
> test but it was enough to convince me to remove the mod.  I=20
> did not look
> at the more esoteric effects such as harmonics or spurs.  I=20
> like a "hot"
> receiver.  If it were me, I would remove the mod and go back to the
> original values.  Jim N4BE
>=20
> Tom Warren wrote:
> >=20
> > Anybody got clues to the intended and non-intended effects=20
> of Field Change
> > No. 7 to the R390A?  This is the change where R702 (screen=20
> resistor on the
> > PTO) and R210 (screen resistor on the 1st crystal=20
> oscillator) are changed
> > from 56K to 220K, presumably to reduce radiation to the=20
> outside world (so
> > the enemy can't listen for your PTO freq or your 1st=20
> oscillator freq and
> > then bomb in that direction).  My guess is that the first=20
> consequence is to
> > reduce the output level of each of those oscillators. =20
> Secondly, there may
> > be some harmonic reduction from each of those oscillators. =20
> However, what
> > else happens?  That is, is the sensitivity of the receiver=20
> or the intermod
> > performance reduced by the modifications?  Also is the=20
> birdy response
> > affected by the modifications (e.g., less harmonic output from those
> > oscillators leads to fewer internal mixer products).  Any=20
> other effects that
> > folks have definitely found?
> >=20
> > My reason for asking is that I'm working on a Motorola PTO=20
> with the 56K
> > screen resistor and a later Cosmos PTO with the 220K screen=20
> resistor and
> > wondering if I ought not to "re-convert" the Cosmos PTO to=20
> have a 56K screen
> > resistor.  R701, R702, and R703 are out of spec on the=20
> Cosmos anyhow, so I
> > will probably change them out, but if a better choice for=20
> R702 is 56K
> > considering that I'm not worried about the enemy direction=20
> finding on my
> > oscillator outputs, then maybe that's what I ought to do.
> >=20
> > For Tom Bridgers and Al Parker, yes, the Motorola PTO is=20
> from the Charlie
> > Taylor (hello Charlie, if you're out there) 390A.  AND I'VE=20
> GOT THE CHARLIE
> > 390A MOSTLY WORKING. HOORAY AFTER ABOUT TWO YEARS.
> >=20
> > Tom, W4PG
> >=20
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>=20